The present invention relates to an injection compacting composition for preparing sintered bodies which comprises an inorganic powder and a specific organic binder. More particularly, the present invention relates to the injection compacting composition which has a good injection compactibility and a good binder removability from green compacts in the sintering step and which can provide sintered bodies having no or little defects such as warpage and cracks and a good dimensional stability in high yields, and also relates to sintered bodies prepared therefrom.
Sintered bodies have been prepared by compacting a mixture of an inorganic powder with an organic binder and sintering the resulting compact. In recent years, ceramic products prepared by mixing a ceramic powder and an organic binder, imparting a plasticity to the mixture, injection-compacting the mixture to give a green compact, removing the binder from the compact and sintering the compact, are being gradually used as engine parts for automobiles. The ceramic products have features that parts of complicated shape can be mass-produced industrially.
The organic binders have connection with uniform dispersibility of ceramic powders, flowability of the powders mixed therewith, strength of green compacts and easiness in removal of the binders from green compacts (binder removability). Accordingly, if the selection and the amount of use of the binders are not appropriate, no good sintered bodies are obtained. For example, if a ceramic powder is not sufficiently uniformly dispersed in an organic binder, defects such as warpage and cracks are easy to be produced in the produced compacts or sintered bodies, and the strength of the compacts is also decreased. Also, if the organic binders are poor in thermal stability, they are deteriorated in the cylinder of an injection molding machine and the flowability of compacting compositions is not stabilized, thus no good injected compacts are obtained or at times the ceramic powder may be separated from the organic binder. If the organic binder is used in excess, it is difficult to avoid blistering and cracking due to a large guantity of a gas generated from the inside of compacts in the step of removing the binder.
As the organic binder used in compacting ceramic powders, there have been used, for instance, waxes and polymers such as ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEA), polystyrene, atactic polypropylene (APP), polyethylene and acrylic and methacrylic polymers. The organic binders used in the preparation of sintered bodies are required to have good characteristics such as compactibility (e.g. compacting stability, mold releasing property and powder flowability in compacting inorganic powders to which the binders are added), strength of green compacts, removability of binder from green compacts by thermal decomposition prior to the sintering, shape retainability in sintering and amount of residual carbon. However, known binders have both merits and demerits and are not always satisfactory in these characteristics.
In general, the polymer binders are superior in compactibility, but have the drawback of being poor in removability. For instance, ethylene-vinyl acetate copolymer (EVA) is superior in powder flowability and can provide green compacts having a high strength and an adequate elasticity without impairing the flowability when mixed with a ceramic powder, but defects such as blister and cracks are easy to occur due to acetic acid gas which starts to generate at near 200.degree. C. in removing the binder from compacts by thermal decomposition.
Similarly to EVA, the ethylene-ethyl acrylate copolymer (EEA) can also provide a high strength and an adequate elasticity to compacts without impairing the flowability when mixed with ceramic powders or metallic powders. However, if it is used in a large quantity, blisters and cracks are markedly produced in the compacts when removing the binder by thermal decomposition. Thus it is difficult to remove the binder without impairing the compacts.
Also, polystyrene, and acrylic and methacrylic polymers (e.g. polyisobutyl methacrylate) have an excellent binding effect on ceramic powders and metallic powders and impart a high strength to green compacts. In particular, they are effective for preventing damages of thin portions, thus imparting an excellent shape retainability to green compacts. They are also superior in thermal decomposability and are easy to be removed from the compacts. However, when they are used in a large quantity, mixtures with ceramic or metallic powders are insufficient in flowability, and are easy to invite poor results in injection molding such as shortage of filling and weld line.
Atactic polypropylene shows a good flowability of mixtures with ceramic powders, and the injection molding is easy and there can be produced green compacts having a strength sufficient for handling (Japanese Patent Publication Kokoku No. 51-29170). However, it has the disadvantage of being inferior in binder removability.
Wax binders such as paraffin are generally superior in removability to polymer binders, but have the drawbacks that they are poor in injection compactibility because the flowability is unstable under a high pressure, and that the strength of obtained green compacts is low because they are compounds with relatively low molecular weight.
In order to obtain a binder having satisfactory properties, it has been required to use a mixed binder by combining two or more of binders having different characteristics in a good balance. However, a difficulty is encountered in obtaining a homogeneous mixture by mixing such known binders, or a long time is required for the mixing, because they differ from each other in form and softening point and also because the solubility or compatibility is not satisfactory. In general, if the mixing of two or more binders is not so sufficient as to provide a homogeneous mixture, it takes a longer time to determine the compacting conditions since the flowability of the mixture is not stabilized. Also, since the uniformity of obtained compacts is impaired, the use of nonuniform binder mixture may cause cracking of sintered bodies in addition to influence on dimensional stability of compacts.
It is an object of the present invnetion to provide an injection molding composition which is suitable for the preparation of sintered bodies of an inorganic powder such as ceramic powder or metallic powder by injection molding, and which has well balanced characteristics such as injection compactibility, strength of green compacts and binder removability by thermal decomposition prior to the sintering, and which can provide in a high yield sintered bodies having no defects such as warpage, cracks and sink marks.
Another object of the present invention is to provide a sintered body of a ceramic powder or a metallic powder having no defects.
These and other objects of the present invention will become apparent from the description hereinafter.